In the realm of battery technology there is a continued need for new and improved cathode and anode materials for use with advanced generation lithium batteries with greater energy densities and improved cycle life properties. In most of the conventional lithium-ion systems, graphitic anodes are used as an intercalation host for lithium to form LiC6. An extensive number of experimental approaches have been proposed to increase the electrode's electrochemical capacity including for example the alloying of lithium with metals such as tin (Sn). The first commercially available amorphous tin-based composite anode was introduced to the market by Sony in 2006, followed by studies of Sn0.34Co0.19C0.47 systems which demonstrated the capabilities of these electrodes in high-energy Li-ion battery systems.
Unfortunately, one of the major obstacles to the use of pure tin as an active material is its large volumetric expansion during the alloying with lithium, which causes cracking and mechanical disintegration of Sn particles. Loss of mechanical and electronic integrity of the active materials leads to severe degradation of the composite anode upon cycling and dramatically shortens the cycle life of the electrode.
Several approaches have been proposed to overcome this problem, including the use of tin oxide as a soft matrix to ameliorate the expansion of the metal. Other approaches have included the use of tin alloys, minimizing the thickness of the electrode, as well as reducing the particle size and uniform particle distribution within the supporting matrix. Some of the fabrication techniques used have include autocatalytic deposition, ball milling, chemical reduction, electro deposition, pyrolysis, sputtering, plasma laser deposition, partial reduction, and vapor deposition. However, these processes often involve several time consuming steps and offer only limited control of particle size and distribution. Moreover, typical Sn and Sn-based alloy anodes are prepared using conventional lamination methods which involves slurry preparation (grinding, mixing with binders, and solvents), laminating, drying, heating, etc.
In our co-pending patent application entitled Graphitized Carbon Coatings for Composite Electrodes, Ser. No. 11/915,837, filed Nov. 28, 2007, which application is commonly assigned and is incorporated herein by reference in its entirety, we described a new process in which microwave plasma chemical vapor deposition (CVD) is used to form graphitic carbon films over substrates at low temperatures. In the microwave plasma CVD process, energy is transmitted directly to a target material through direct interaction between the microwaves and the molecules of the material. Thus film deposition rate is fast, and can reach several tens of microns per minute. According to various embodiments of that invention, it was found that by using microwave technology, highly graphitized carbon films with good conductivity could be produced without significant heating of the substrate.